The long-term objective of this proposal is to test the hypothesis that the muscles that protrude and retract the tongue (genioglossus and hypoglossus/styloglossus muscles, respectively) are co-activated during inspiration, and that co-contraction contributes significantly to the maintenance of pharyngeal airway patency. The conceptual model is that co-contraction during inspiration stiffens the tongue as the antagonist muscles work against one another, thereby minimizing backward displacement of the tongue and subsequent occlusion of the pharynx. Significant new data showing respiratory-related co-activation of the protrudor and retractor muscles in animal models, as well as recent evidence showing improved inspiratory airflow with co-activation in human subjects with obstructive sleep apnea, provide strong support for this conceptual framework. Accordingly, the following Specific Aims are designed to rigorously test the co-activation hypothesis using an anesthetized rat model: Aim 1 is to demonstrate that the protrudor and retractor muscles of the tongue are co-activated during breathing and that they respond similarly to changes in respiratory related stimuli. Aim 2 is to show that co-activation of the extrinsic tongue muscles will improve pharyngeal airway mechanics more than the independent activation of either the protrudor or retractor muscles. Aim 3 is to demonstrate that the initial operating length of the tongue muscles will influence: a) the magnitude of respiratory related tongue movements, b) the ability of the tongue muscles to modulate pharyngeal airway flow mechanics, c) the fatigability of the tongue muscles. These experiments will lay the foundation for new and improved treatment strategies for persons with obstructive sleep apnea or with other conditions that are caused by malfunction of the tongue motor system.